1. Field of the Invention
This invention relates to semiconductor integrated circuit (LSI) device contained switch circuits and variable capacitor elements and a frequency modulation device.
2. Description of the Related Art
Bluetooth™ is proposed as a new standard of radio communications for mobile terminals in order to exchange data items between mobile telephones, personal digital assistants (PDA), notebook-sized personal computers and voice terminal devices such as headphones. In Bluetooth™, a carrier signal with a frequency of 2.4 GHz band is used and, for example, a transmission speed of 1 Mbps at maximum is realized.
In a mobile terminal (which is hereinafter referred to as a Bluetooth device) which can cope with Bluetooth™, 79 frequency channels defined in the frequency band of 2.402 GHz to 2.480 GHz are used. A frequency hopping system is used in which radio communication is performed by changing the frequency channels with time. In the frequency hopping system, channel selection is repeatedly made at preset time intervals based on a predetermined pseudo random algorithm and one packet data is allocated to one channel to perform communication.
As one example of a frequency shift key (FSK) modulation device used in the radio communication system of the frequency hopping system, for example, a device disclosed by H. ISHIKURO et al., “A Single-Chip CMOS Bluetooth Transceiver with 1.5 MHz IF and Direct Modulation Transmitter”, ISSCC 2003 SESSION 5 WIRELESS-PAN TRANSCEIVERS PAPER 5.5, DIGEST OF TECHNICAL PAPERS 95 is known. In the FSK modulation device, a VCO direct modulation system is provided in which a carrier signal with a preset frequency is generated from a voltage controlled oscillator circuit (VCO) and the frequency of the carrier signal is shifted according to “1”, “0” of a transmission data signal input to the VCO.
An error signal is generated by comparing the carrier signal generated from the VCO with a reference signal and control voltage which is used to control the oscillation operation of the VCO is generated by supplying the error signal to a charge pump circuit. The charge pump circuit which generates the control voltage based on the error signal is normally configured by MOS transistors.
In recent years, a leak current of the MOS transistor increases with miniaturization of the LSI element and scaling of the MOS transistor. The leak current (gate-induced drain leakage current: GIDL current) at the OFF period of the MOS transistor tends to have a high degree of dependence on the drain-substrate voltage (Vdb) with scaling of the MOS transistor. For example, as shown by the characteristic indicated by broken lines in FIG. 23, the leak current (Id) increases. Further, a leak current caused by a drain-induced barrier lowering has a high degree of dependence on the drain-source voltage (Vds).
Further, if the leak current of the MOS transistor in the charge pump circuit is large when an output node of the charge pump circuit is set in an electrically floating state, the control voltage varies with time and a frequency drift of the output signal of the VCO occurs so as to make it difficult to perform data transmission with high reliability. Incidentally, in the specification of the Bluetooth device, the maximum packet length 3 ms, the frequency shift amount of the transmission signal is ±160 kHz, for example, and the frequency drift amount of the transmission signal is ±40 kHz or less.
The problem associated with the leak current described above does not occur only in the charge pump circuit. Generally, when a leak current of a switch circuit connected to a node which is desired to be set into an electrically floating state in a preset period in the LSI is large, a problem that the node voltage will vary occurs.
In order to cope with the above problem, a capacitor element with large capacitance is connected to the node so as to suppress the potential variation to a small value even when the current leak occurs. However, since the area of the capacitor element of large capacitance is large, the chip cost rises.
Further, in the FSK modulation device of the VCO direct modulation system using the conventional switch circuit, there occurs a problem that an OFF-state leak current flows in the MOS transistor configuring the switch circuit and, as a result, a frequency variation occurs in the carrier signal generated.
In addition, the problem associated with the leak current in the node set in the floating state described above also occurs in a variable capacitor element to be formed in the LSI device in the same way.
In Jpn. Pat. Appln. KOKAI Publication No. H8-213909, the technique for performing the control operation to eliminate the difference between potentials on the input side and output side of switch means at the OFF time of the switch means.